What is the best minimal runtime for d?

[Taylor Hillegeist via Digitalmars-d-learn]

I've been playing around with d with a KL25Z eval board. However 
it is not easy, It's not easy to know what features are and are 
not usable. when will i get a linker error to some 
__eabi_something_not_in_the_runtime.

So, question is, does there exist a minimal runtime that will 
work with LDC/GDC and is up to date?


Also I find it interesting that its hard to distinguish what 
features are language features and which are run-time features. 
If that is even on someones radar.

Also bonus question:
How do you think should registers be read and written to in D?